I'm sorry to hear about your new diagnosis. Has your doctor said whether a portable oxygen concentrator is appropriate for you? If so, that's the way to go when flying (you can rent one if you don't want to buy). You can't bring your own O2 on the plane, so have your local company help set you up with a company in Orlando. Also, you may need paperwork for the airline from your doctor's office. I would check Southwest's website soon so you have plenty of time to take care of that. My FIL traveled (by air) around the country with O2 for years. Generally it went smoothly. Both his doctor and his O2 supplier will well versed in setting up what he needed for travel.
Also, be sure to note that the thrill rides at WDW probably won't work for you, but other rides and shows should be fine, especially if you have a small tank for the day while you're out in the park. Also, I believe the First Aid station will store other small tanks for you, if you need to swap them out during the day. I think you can drop your tanks off in the AM and then stop in at First Aid as needed. FIL preferred the small tanks when not on a plane, but they wouldn't have lasted an entire day at WDW.
Good luck and best wishes for your trip!
Mary